Preheat barbecue grill (oil hot grill to help prevent sticking).
Peel and devein the shrimp, leaving tails on.
Using a sharp knife, butterfly shrimp by cutting along the inner curve or underbelly about halfway (almost completely through), leaving the tails firmly attached.
Press the shrimp down flat like a butterfly.
Place prepared shrimp in a shallow dish and sprinkle with olive oil; refrigerate, covered, until ready to cook.
In a medium bowl, combine white truffle oil, lemon juice, onion, and parsley.
Season to taste with salt and pepper; set aside.
When ready to cook, season the shrimp lightly with salt and pepper.
Place shrimp onto hot grill and cook approximately 2 minutes on one side; turn and cook another 1 minute or until shrimp are opaque in center (cut to test).
Remove from grill.
To serve, arrange shrimp (split sides up) on a serving plate.
Drizzle with a little of the vinaigrette.
Place remaining vinaigrette in a small bowl and place on plate for dipping.